U.S. Pat. No. 4,033,216 describes a punch having a front side and a rear side comprising a U-shaped machine frame with an upper leg, a lower leg and a crosspiece, which forms a U-shaped work space open on the front side of the punch machine but closed on the rear side by the crosspiece between the upper and the lower leg.
A punch piston-cylinder unit with a punch tool is mounted in the upper leg and a die-bearing member and/or a workpiece receiving member are mounted in the lower leg.
At least the upper leg experiences a bending moment due to punching or stamping forces occurring during a punching operation. Hence, an elastic deformation of that leg occurs.
In the punch machine of U.S. Pat. No. 4,033,216 according to different punching forces occurring in working different weight workpieces, the leg of the machine frame experiences different bending moments. The machine frame and the leg are designed so that the maximum occurring bending moment can be withstood by the frame and is not particularly troublesome. However that makes the machine frame heavy. Furthermore any deformation of the frame occurring causes a deviation of the punch tool axis and the axis of the die-bearing member from their initial axial parallel relationship.
To limit the bending moment occurring in a punch machine of the described basic structure, a strap is installed at the front end of a leg, just in front of the punch piston-cylinder unit, which is pivotable about a horizontal axis and in a pivoted-in state connects the front of the upper leg and the lower U-leg. During the punching process the strap is under tension. This particular design from the connecting member or strap is troublesome because in the pivoted-in state the work space is no longer easily accessible and open.
In DE-OS No. 2926097 a different method is used to limit the bending moment in a punch machine. Here a second machine frame is movable in a horizontal direction in the first machine frame, which carries the punch piston-cylinder unit with the punch tool and the die-bearing member. That is comparatively expensive. Since the deformation of the legs can destroy the parallel relationship between the punch tool axis and the die-bearing member axis, as described in DE-OS No. 24 42 433 the punch piston-cylinder unit with the punch tool is supported pivotally on the upper leg so that during the punch process the parallel relationship between the punch tool and the opposing die-bearing member is maintained.